In bundled drawing of stainless steel fibers a number of stainless steel wires are bundled and drawn together. The individual wires are separated from one another by covering each stainless steel wire, possibly even on wire rod diameter, with a suitable matrix material. All stainless steel wires, covered with matrix material, are enveloped in an envelope material. Once the bundle of enveloped wires, also called the composite wire, is drawn to the desired diameter, the envelope material and the matrix material are removed, usually by leaching. Very often a metal such as iron or copper is used as matrix and/or envelope material. The use of such metal as matrix material is advantageous since a metal has similar deformability properties as the stainless steel wire that has to be drawn into stainless steel fibers. The metal matrix material is compatible with the stainless steel wires during the drawing and annealing operations. The metal matrix material has a lower chemical resistance and allows the stainless steel fibers to be freed from the matrix material in a leaching process quite easily. An important drawback of using a metal as matrix material is the mutual solubility of stainless steel and matrix material that may be observed during heat treatments. This drawback is observed especially with stainless steels that have quick cold work hardening and therefore require frequent heat treatments e.g. AISI 302.
Intermediate heat treatments, performed between two drawing steps, result in a diffusion of elements of the matrix material into the stainless steel wires and/or in a diffusion of the elements of the stainless steel wires into the matrix material. This has as consequence that the composition of the steel may be changed to some extent after a heat treatment. This effect is most pronounced at the surface of the stainless steel fibers.
Differences in the composition of the stainless steel due to diffusion may cause unreliability of the properties of the stainless steel fibers, for example in the electrical and chemical properties or in the behavior of the stainless steel fibers exposed to high temperatures.
Prior art provides only one solution to the drawback of inhomogeneous surface composition of stainless steel fiber, being the use of electrochemical leaching as process for removing the matrix material as described in EP337517A1. This method is not industrially attractive due to excessive investment costs, causing significant cost price increase of the fibers so obtained.
Another consequence of the diffusion is that more matrix material is necessary in order to assure a separation of the stainless steel fibers during manufacturing of the stainless steel fibers.